1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device, and more particularly, to an organic EL display device which includes at least two driving transistors and an organic light emitting diode having at least two first electrodes and a common second electrode per pixel in order to operate the pixel due to a short circuit occurring between one of the first electrode and the second electrode
2. Description of the Related Art
An organic light emitting diode is an emissive device which emits fluorescent light by recombining electrons supplied from a cathode and holes supplied from an anode. An EL display device, which employs the organic light emitting diode, does not need a separate backlight, in contrast with a liquid crystal display (LCD) device, and has a wide viewing angle, fast response speed, low DC driving voltage, and light weight as compared to a passive light emitting diode. Thus, the EL display device is suitable for use as a wall-mountable display device or a portable display device.
Methods of driving an organic EL display panel of an EL display device include a passive matrix driving method and an active matrix method using a thin film transistor (TFT). In the passive matrix driving method, an anode and a cathode are formed to be perpendicular to each other, and the display panel is driven by selecting a line. In the active matrix driving method, the TFT is connected to an anode, e.g., indium tin oxide (ITO), and the display panel is driven according to a voltage maintained by a capacitor connected to a gate of the TFT.
FIG. 1 is a block diagram of a conventional organic EL display device.
Referring to FIG. 1, the organic EL display device includes a display panel 10, a data driver 20, and a scan driver 30.
The display panel 10 includes a plurality of data lines D1 to Dm for transmitting a data signal(s), a plurality of scan lines S1 to Sn for transmitting a selection signal(s), which are perpendicular to the plurality of data lines D1 to Dm, and a plurality of pixel circuits P11 to Pnm formed at crossing points of the plurality of data lines D1 to Dm and the plurality of scan lines S1 to Sn.
The data driver 20 outputs the data signal representing an image signal through the plurality of data lines D1 to Dm, and the scan driver 30 sequentially outputs the selection signal to the pixel circuit P11 to Pnm through the plurality of scan lines S1 to Sn.
FIG. 2 is a circuit diagram of one pixel of a conventional organic EL display device, i.e., one representative pixel among N×M pixel circuits in the display panel 10 of the organic EL display device of FIG. 1.
As shown in FIG. 2, the pixel circuit 11 includes an organic light emitting diode OLED, a switching transistor M1, a driving transistor M2, and a capacitor Cst.
The switching transistor M1 has a gate connected to the scan line Sn and a source connected to the data line Dm, and transmits the data signal from the data line Dm to a gate of the driving transistor M2 in response to the selection signal from the scan line Sn.
The driving transistor M2 has a source connected to a power voltage Vdd, and the gate connected to a drain of the switching transistor M1, and the capacitor Cst is connected between the gate of the driving transistor M2 and the power voltage Vdd. The capacitor Cst maintains a gate-source voltage VGS of the driving transistor M2 for a predetermined time period.
The organic light emitting diode OLED has an anode a connected to a drain of the driving transistor M2 and a cathode b connected to a reference voltage Vss and emits light corresponding to a driving current applied through the driving transistor M2. Here, the reference voltage Vss connected to the cathode b of the organic light emitting diode OLED is lower than the power voltage Vdd and may be, for example, the ground voltage.
Here, the driving current which flows through the organic light emitting diode OLED is given by the following Equation 1:
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                                    β              2                        ⁢                                          (                                  VDD                  -                  Vdata                  -                                                          Vth                                                                      )                            2                                                          Equation        ⁢                                  ⁢        1            where IOLED is the driving current which flows through the organic light emitting diode OLED, Vgs is a voltage between the gate and the source of the driving transistor M2, Vth is a threshold voltage of the driving transistor M2, Vdata is a data voltage, and β is a constant.
As can be seen in Equation 1, the driving current corresponding to the data voltage Vdata applied by the pixel circuit 11 of FIG. 2 is supplied to the organic light emitting diode OLED, and the organic light emitting diode OLED emits light in response to the supplied current.
FIG. 3 is a plan view of one pixel of a conventional organic EL display device.
Referring to FIG. 3, the pixel includes a scan line 32 arranged in one direction, a data line 31 arranged perpendicular to the scan line 32, and a power voltage line 37 arranged perpendicular to the scan line 32 and parallel to the data line 31. A switching transistor 33 is connected to the data line 31 and the scan line 32. A capacitor includes a lower capacitor electrode 35 connected to a source or drain electrode 34 of the switching transistor 33 through a contact hole, and an upper capacitor electrode 36 arranged above the lower capacitor electrode 35 to be connected to the power voltage line 37. A gate 38 of a driving transistor 39 is connected to the lower capacitor electrode 35, and an anode a is connected to a source or drain electrode 40 through a via hole 41.
In a unit pixel of the conventional organic light emitting diode described above, an anode a is formed on a substrate, an organic emission layer is formed on the anode a, and a cathode b is formed on the organic emission layer.
Only the organic emission layer (see EL in FIG. 4) is formed between the anode a and the cathode b, and an insulating layer is formed around the anode a, thereby preventing the anode a and the cathode b from being electrically connected directly without the emission layer.
However, the conventional organic light emitting diode has one anode and a common cathode per unit pixel; and a relationship between the anode and the cathode may be affected by fine particles inserted between the anode and the cathode during a manufacturing process, a pattern failure of a lower layer, and/or an external pressure. For the foregoing reasons, the anode and the cathode, which should be electrically insulated from each other, may be electrically connected as shown in FIG. 4. FIG. 4 is a photograph illustrating a short circuit formed between the anode and the cathode in the conventional organic EL display device. In FIG. 4, “a” denotes the anode, “b” denotes the cathode, and “c” denotes the fine particles.
As shown in FIG. 4, the fine particles c exist in the insulating layer between the anode a and the cathode b, thereby causing an electrical short circuit between the anode a and the cathode b. The short circuit between the anode a and the cathode b leads to an application of a cathode voltage Vss to the anode a. As a result, a driving current (e.g., IOLED) of a driving transistor (e.g., M2) according to a data signal flows not to the organic emission layer but to the cathode b, so that light of a predetermined color is not emitted, thereby causing a dark pixel, i.e., a pixel defect. As organic EL display devices become more compact, more dark pixels resulting from short circuits between the anode and the cathode may occur. Therefore, it is desirable to resolve the foregoing problems.